1. Field of the Invention
The present invention relates to disc drive storage systems.
2. Discussion of the Related Art
Disc drives are used in many different types of computer or data processing systems to store data. Disc drive systems include one or more discs of a recording medium (e.g., a magnetic recording medium or an optical recording medium) on which information can be written for storage purposes, and from which stored information can be read. The recording medium is typically in the form of a disc 1 as shown in FIG. 1. The disc includes a plurality of tracks on which information is recorded. In FIG. 1, only an outer track 3 and an inner track 5 are shown to simplify the drawing. However, it should be appreciated that a surface of an actual recording disc will include a large number of tracks (e.g., a 9 GB drive includes over 5,000 tracks) in addition to the outer and inner tracks 3, 5. In a disc drive that includes multiple discs, the discs are conventionally stacked so that corresponding tracks on the discs overlie one another.
As shown in FIG. 1, each of the tracks is conventionally subdivided into a plurality of sectors 7 (also know as blocks). The sectors 7 define the smallest amount of data that is written to or read from the disc in one operation. An exemplary size for each sector is 512 bytes, which is the standard for disc drives that communicate with other components of a data processing system over a SCSI interface.
Data is read from and written to the disc 1 using a head 9 that is positioned adjacent (e.g., above) the surface of the disc via an arm 11. In operation, the disc is rotated at a high rate of speed (e.g., 5,400 rpm, 7,200 rpm or 10,033 rpm). The arm 11 is pivoted by an actuator (not shown) about a pivot point 12 to move the head 9 in a seek direction (indicated by the arrow S in FIG. 1) so that the head can be positioned above any of the tracks 3, 5 of the disc. The combination of the rotation of the disc and the movement of the head 9 in the seek direction S enables the head to be positioned adjacent any sector 7 of the disc to access (i.e., read information from or write information to) that sector.
The performance of a disc drive system is largely impacted by three system characteristics, i.e., seek time, latency and data rate. The seek time relates to the delay incurred in positioning the head 9 above the appropriate track. In the worst case, the seek time is defined by the delay incurred in moving the head 9 between the inner and outer tracks 5, 3. The latency of the system is the time it takes for the rotation of the disc 1 to bring the desired sector 7 to a position underlying the head 9. The worst case latency is defined by the time it takes to complete a single rotation of the disc. Finally, the data rate of the system relates to how quickly data can be read from or written to the disc once the head 9 is positioned above the appropriate sector 7. The data rate is dependent upon the bit density of the information stored on the disc, the rate of rotation of the disc and the disc drive electronics that process the data.
Most conventional disc drive systems attempt to maximize the storage capacity of the disc 1. Thus, the disc 1 is typically provided with as many tracks as possible, and each track is provided with as many sectors as possible. Although maximizing storage capacity, such configurations result in limitations being placed on the performance of the system. For example, maximizing the number of tracks on the disc results in a long worst case seek time, because the head 9 must move across substantially the entire radius of the disc.
In addition, the relative performance of the disc drive system is greater when accessing tracks that are positioned nearer the outer surface of the disc (e.g., track 3) than the center of the disc (e.g., track 5). Many disc drive systems employ a technique known as zoned constant velocity in which the total disc capacity is increased by varying the number of sectors per track with the distance of the track from the center of the disc. This technique is also called zone bit recording. A drive that employs this technique is known as a notched drive according to the SCSI specification. The tracks are typically grouped into zones with each track in a zone including the same number of sectors. Outer tracks have more sectors than inner tracks. As a result, when the disc rotates, the rate of data passing by the head when accessing the outer track 3 is significantly greater than when accessing the inner track 5, because the outer track moves past the head at a significantly faster speed and has more sectors per track. Thus, the data rate of the system is greater when reading a track positioned near the outer surface of the disc. In addition, since the outer track 3 may have relatively more information stored thereon, less seeking between tracks is required when accessing the outer tracks.
As should be appreciated from the foregoing, when the entire surface of the disc is used to support tracks in a conventional implementation as shown in FIG. 1, the performance of the disc drive system is limited by the poorer performance of the system when accessing the inner tracks.
In view of the foregoing, it is an object of the present invention to provide an improved method and apparatus for increasing disc drive performance.